1. Field of the Invention
The present invention relates to an optical device such as an optical switch or a variable optical attenuator provided with optical elements such as mirrors, prisms, or shutter used in the field of optical communications as optical information networks, or optical LAN.
2. Related Art
So far, as this type of optical devices, an optical switch 900 is known as shown in, for example, FIG. 10.
The optical switch 900 is a 2×2 optical switch of two inputs and two outputs having optical fibers 911, 921 and lenses 912, 922 to form input ports 910a, 920a, and having fiber collimators 910, 920 for making passing light beams to be parallel, optical fibers 931, 941 and lenses 932, 942 to form output ports 930a, 940a and having fiber collimators 930, 940 for making passing light beams to be parallel.
Further, the optical switch 900 is provided with an optical path switching element 950 having right-angled prisms 951, 952, a support plate 960 for supporting the optical path switching element 950, a drive actuator 970 such as a solenoid actuator for moving the optical path switching element 950 in directions as shown by arrows 950a and 950b, a guide mechanism 980 for guiding the optical path switching element 950 moved by the drive actuator 970, and a coupling mechanism 990 for connecting the drive actuator 970 and the guide mechanism 980.
The optical switch 900 moves the optical path switching element 950 in the direction shown with the arrow 950a by the drive actuator 970 and draws it out between the fiber collimators 910, 920 and the fiber collimators 930, 940, thereby to allow light beams output at the fiber collimators 910, 920 to enter directly into the fiber collimators 930, 940.
On the other hand, the optical switch 900 moves the optical path switching element 950 in a direction shown with the arrow 950b by the drive actuator 970, and inserts it between the fiber collimators 910, 920 and the fiber collimators 930, 940, so that, as shown in FIG. 10, the light beams output at the fiber collimators 910, 920 are reflected twice by the right-angled prisms 951, 952 of the optical path switching element 950 to be received by the fiber collimators 940, 930, respectively.
Namely, in dependence upon inserting the optical path switching element 950 into or retracting from the position between the fiber collimators 910, 920 and the fiber collimators 930, 940, the optical switch 900 may switch the optical paths so that light beams output at the fiber collimator 910 and the fiber collimator 920 are received by the fiber collimator 930 and the fiber collimator 940 respectively, or so that the optical paths through which the light beam output at the fiber collimators 910, 920 are received by the fiber collimator 940 and the fiber collimator 930 respectively.
Incidentally, in the optical switch 900, it is desirable that the insertion loss is small in case the optical path switching element 950 is absent in the optical path as well as in case the optical path switching element 950 is located for switching the optical path.
In order to reduce the insertion loss when switching the optical paths, in case the optical path switching element 950 is arranged in the optical path, it is required to reduce the loss of the light beams when receiving the light beams output from the fiber collimators 910, 920 forming input ports 910a, 920a by the fiber collimators 930, 940 forming output ports 930a, 940a. 
Under a condition that the optical path switching element 950 is drawn out from between the fiber collimators 910, 920 and the fiber collimators 930, 940, in case the light beams output from the fiber collimators 910, 920 are received by the fiber collimators 930, 940, the insertion loss may be reduced by decreasing dislocation of an optical axis from the fiber collimator 910 and fiber collimator 930 and by decreasing the dislocation of the optical axis from the fiber collimator 920 and fiber collimator 940.
Further, under a condition that the optical path switching element 950 is inserted between the fiber collimators 910, 920 and the fiber collimators 930, 940, in case the light beams output from the fiber collimators 910, 920 are received by the fiber collimators 930, 940, the insertion loss may be reduced by decreasing the dislocation from the light beam 900a output from the fiber collimator 910 and reflected twice by the right-angled prism 951 and from the optical axis of the fiber collimator 940 and by decreasing the dislocation from the light beam 900b output from the fiber collimator 920 and reflected twice by the right-angled prism 952 and from the optical axis of the fiber collimator 930.
Herein, in case the optical path switching element 950 is not located in the optical path, dislocations of optical axes from the fiber collimator 910 and the fiber collimator 930, or from the fiber collimator 920 and the fiber collimator 940 may be adjusted by controlling and fixing positions and angles of the fiber collimators 910, 920, 930, 940. But, dislocations of the optical axes from the light beam 900a and the fiber collimator 940, or from the light beam 900b and the fiber collimator 930 cannot be adjusted, unless adjusting the positions and the angles of the optical path switching element 950 in addition to the positional adjustment of the fiber collimators 910, 920, 930, 940.
Namely, under the condition that the optical path switching element 950 is inserted into between the fiber collimators 910, 920 and the fiber collimators 930, 940, the light beams output from the fiber collimators 910, 920 are received by the fiber collimators 930, 940 through the optical path switching element 950. Therefore, in comparison with the condition that the optical path switching element 950 is drawn out from between the fiber collimators 910, 920 and the fiber collimators 930, 940, it is difficult to reduce the loss of the light beams output at the fiber collimators 910, 920 when entering into the fiber collimators 930, 940.
The invention will be described more in detail in reference to FIG. 11. In the same, the angle 951a of the right-angled prism 951 is 90° and the angle 951b is 45° . A ridgeline of a portion of the angle 951a of the right-angled prism 951 is set as a z-axis, the optical axis of fiber collimator 910 is set as an x-axis, and the rotation angles around the x, y and z axes are set as the rotation angles θ, φ and φ respectively.
In case the light beam 900a output from the fiber collimator 910 and reflected twice by the right-angled prism 951 is dislocated as shown by Δφ in a direction of rotation angle φ from an angle designed by the optical path switching element 950, there occurs angular dislocation from the optical axis 940b of the fiber collimator 940.
Further, although not illustrated, in case the light beam 900a is dislocated from the position designed by the optical path switching element 950, or dislocated in the rotating direction θ or in the rotating direction φ from the angle designed by the same, the light beam 900a does not cause angular dislocation from the optical axis 940b of the fiber collimator 940, but causes positional dislocation.
Herein, as to the loss of the light beams output from the fiber collimators 910, 920 when entering the fiber collimators 930, 940, the angular dislocation of the light beam 900a and the optical axis 940 from the optical axis 940b is larger than the positional dislocation of the light beam 900a and the fiber collimator 940 from the optical axis 940b. 
For example, in case a diameter of the light beam 900a is 0.5 mm and a tolerable coupling loss is 0.2 dB, the angular dislocation between the light beam 900a and the optical axis 940b of the fiber collimator 940 is tolerable until 0.05 mm in practically measuring, while the positional dislocation between the light beam 900a and the optical axis 940b of the fiber collimator 940 is tolerable at only 0.02°.
The positional dislocation between the light beam 900a and the optical axis 940b of fiber collimator 940 may be adjusted to 0.01 mm or less by use of a guide mechanism 980 or other positioning instruments. It is, however, extremely difficult to adjust the angular dislocation between the light beam 900a and the optical axis 940b of fiber collimator 940 to 0.02 mm or less in comparison with adjustment of the positional dislocation between the light beam 900a and the optical axis 940b of the fiber collimator 940 is made 0.01 mm or less.
As to the conventional optical switch 900, in order to render the angular dislocation between the light beam 900a and the optical axis 940b of the fiber collimator 940 to 0.02 mm or less, a mechanism called as a precision guide structure is employed, which is disposed, in a guide mechanism 980, between a guide member having more than two fixed guide surfaces and a slider arranged between the more than two fixed guide surfaces and carrying the optical path switching element 950 as keeping a space from the guide member.
However, according to the optical switch 900, there are problems that, when switching the optical path, it is difficult to insert the optical path switching element 950 into the optical path at a high angular precision with reduction of the insertion loss.
Namely, the precision guide structure has been involved with problems that it is required that the gap between the fixed guide surface and the slider is designed and processed to be 0.001 mm or less for regulating the angular dislocation of the optical path switching element to be 0.02° or less, bringing about cost-up and less endurance. Further, when the slider moves, the fixed guide surfaces and the slider surfaces are worn to reduce dimensions. In short, the gap between the two fixed guide surfaces and the slider is widened and mechanical play is created. The mechanical play enlarges an inclining angle of the optical path switching element, increase the loss of light, and weakens characteristics of the optical switch. Endurance against number of movements of the slider is required.
As a structure other than a slide bearing structure such as the precision guide structure, a roller bearing structure using the bearing may be considered, but since bearing balls or rods are worn owing to ten thousands of movements of the slider, it is extremely difficult to maintain the precision of the optical path switching element and the angle of the slider being 0.02° or less.